The current fourth generation (4G) wireless access within the 3rd generation partnership project (3GPP) long-term evolution (LTE) is based on orthogonal frequency-division multiplexing (OFDM) in downlink and discrete Fourier transform (DFT) spread OFDM, also known as single carrier frequency division multiple access (SC-FDMA), in uplink.
A candidate for a fifth generation (5G) air interface is to scale the current LTE air interface, which is limited to 20 MHz bandwidth, N times in bandwidth with 1/N times shorter transmission-time duration. A typical value may be N=5 so that the carrier has 100 MHz bandwidth and 0.1 millisecond slot lengths. With this approach many functions in LTE can remain the same, which would simplify the standardization effort and allow for a reuse of technology components.
The carrier frequency for an anticipated 5G system could be higher than current 4G systems. Values in the range of 10-80 GHz have been discussed. At such high frequencies, it is suitable to use an array antenna to achieve beamforming gain. Since the wavelength is small, e.g., less than 3 cm, an array antenna with a large number of antenna elements can be fitted into an antenna enclosure with a size comparable to 3G and 4G base station antennas of today.
FIG. 1 is a block diagram illustrating a radio network 100 that includes one or more wireless devices 110A-C, network nodes 115A-C (shown in FIG. 1 as base stations), radio network controller 120, and packet core network 130.
A wireless device 110 may communicate with a network node 115 over a wireless interface. For example, wireless device 110 may transmit wireless signals to network node 115 and/or receive wireless signals from network node 115. The wireless signals may contain voice traffic, data traffic, control signals, and/or any other suitable information.
FIG. 2 is a block diagram illustrating a network 200 that includes three transmission points (TP) 202 for communicating with wireless device 110 or other user equipment (UE) via array antennas generating multiple beams 203. A transmission point may include any network node such as network node 115 shown in FIG. 1.
The beams generated by array antennas may typically be highly directive and give beamforming gains of 20 dB or more due to that a large number of antenna elements participate in forming a beam. This means that each beam is relatively narrow in angle, a half-power beam width (HPBW) of 5 degrees is not unlikely. Hence, a sector of a network node, such as a base station, must be covered with a large number of beams.
Where a system such as system 200 of FIG. 2 includes multiple transmission nodes, each node may have an array antenna capable of generating many beams 203 with small HPBW. These nodes may then, for instance, use one or multiple carriers, so that a total transmission bandwidth of multiples of hundreds of MHz can be achieved leading to downlink (DL) peak user throughputs reaching as high as 10 Gbit/s or more.
In LTE access procedures, a wireless device, or UE, first searches for a cell using a cell search procedure, where unique primary and secondary synchronization signals (PSS and SSS, respectively) are transmitted from each network node, or eNodeB in the context of LTE. When a cell has been found, the wireless device can proceed with further steps to become associated with this cell, which is then known as the serving cell for this wireless device. After the cell is found, the wireless device can read system information (transmitted on the physical broadcast channel), known as the master information block (MIB), which is found in a known time-frequency position relative to the PSS and SSS locations. After MIB is detected, the system frame number (SFN) and the downlink system bandwidth are known.
In LTE, as in any communication system, a mobile terminal may need to contact the network without having a dedicated resource in the uplink (UL) from wireless device to network node, or base station. To handle this, a random-access procedure is available where a wireless device that does not have a dedicated UL resource may transmit a signal to the base station.
FIG. 3 is a block diagram illustrating random-access preamble transmission 300. The first message of this procedure is typically transmitted on a special communication resource reserved for random access, a physical random-access channel (PRACH). This channel can for instance be limited in time and/or frequency (as in LTE).
The communication resources available for PRACH transmission are provided to the wireless devices as part of the broadcasted system information in system information block two (SIB-2) or as part of dedicated radio-resource control (RRC) signaling in case of, e.g., handover.
The resources consist of a preamble sequence and a time/frequency resource. In each cell, there are 64 preamble sequences available. Two subsets of the 64 sequences are defined, where the set of sequences in each subset is signaled as part of the system information.
FIG. 4 is a signaling diagram illustrating a contention-based random-access procedure used in LTE. The wireless device 110 starts the random-access procedure by randomly selecting one of the preambles available for contention-based random access. At step 402, the wireless device 110 transmits a random-access preamble (MSG1) on the physical random-access channel (PRACH) to network node 115.
At step 404, the radio-access network (RAN) acknowledges any preamble it detects by transmitting, from network node 115, a random-access response (MSG2) including an initial grant to be used on the uplink shared channel, a radio network temporary identifier (TC-RNTI), and a time alignment (TA) update. When receiving the response, the wireless device 110 uses the grant to transmit a scheduled transmission message (MSG3) to network node 115 at step 406.
The procedure ends with the RAN resolving any preamble contention that may have occurred for the case that multiple wireless devices transmitted the same preamble at the same time. This can occur since each wireless device 110 randomly selects when to transmit and which preamble to use. If multiple wireless devices select the same preamble for the transmission on PRACH, there will be contention that needs to be resolved through a contention resolution message (MSG4), which may be transmitted in a step 408.
FIG. 4 also illustrates transmissions of hybrid automatic repeat request acknowledgement messages (HARQ ACK).
FIG. 5 is a block diagram illustrating contention-based random access, where there is contention between two wireless devices. Specifically, two wireless devices 110A, 110B, transmit the same preamble, p5, at the same time. A third wireless device 110C also transmits at the same time, but since it transmits with a different preamble, p1, there is no contention between this wireless device and the other two wireless devices.
A wireless device 110 can also perform non-contention based random access. FIG. 6 is a flowchart illustrating the procedure for a wireless device 110 to perform contention-free random access based on reception of a random access (RA) order message from network node 115. Non-contention based random access is typically used in handover between two network nodes, such as any two of the network nodes 115A, 115B, 115C illustrated in FIG. 1. In this case, the order for a non-contention based random access is transmitted from a source network node while the random access preamble (MSG 1) is received at another target network node, which also transmits the random access response (MSG 2). Similar to the contention-based random access, the random-access response (MSG2) is transmitted in the downlink (DL) to the wireless device 110 following successful detection of a random-access preamble (MSG1).